There is a laser annealing method in which a non-crystalline thin film is irradiated with laser light and modified. For instance, in a liquid crystal display (LCD) currently in volume production, an insulated gate thin film transistor (TFT) based on amorphous silicon (a-Si) is used as a pixel switch. However, realization of a display with high functionality such as high definition and high speed is beyond the capability of the TFT based on a-Si having an electric field mobility (μFE) as low as 1 cm2/Vs or less.
On the other hand, in another laser annealing method, a-Si is irradiated with excimer laser to form polycrystalline silicon for use in TFT. In such TFT, in the experimental stage, μFE has attained approximately 100-200 cm2/Vs. Thus, higher functionality such as higher definition and higher speed of display, and integral formation of driving circuits can be expected.
However, the energy density (fluence) of the irradiating laser beam may be varied. Due to this variation, grains outside the desired grain size are formed in the polycrystalline silicon obtained after annealing. This causes the problem of variation in the grain size of polycrystalline silicon. Furthermore, there is no defined value for detecting whether grains outside the desired grain size occur for a particular intensity distribution of the laser beam.